1. Field of the Invention
The present invention relates to a timer device that is employed to adjust the quantity of advance angle in a distributor type fuel injection pump, typical examples of which include a VE type fuel injection pump and a VR fuel injection pump.
2. Description of the Related Art
Timer devices in the prior art include, for instance, the timer device disclosed in Japanese Unexamined Patent Publication No. H 8-42362. In this publication, a timer device for a VR type fuel injection pump is disclosed. This timer device includes a cylindrical steel pipe mounted at a nonferrous pump housing, with a timing piston slidably inserted in the steel pipe. A low pressure chamber provided with a return spring is formed at one end of the timing piston, and a high pressure chamber enclosed by an integrated flange (which, together with the steel pipe, constitutes an integrated unit) is formed at another end of the timing piston passage that communicates with the outlet side of a feed pump is connected to the high pressure chamber.
In addition, since an impact force (drive reactive force) is applied to the timer piston when the rollers ride up the cam lobes, an impact valve and a drain are provided at the timer piston so that any displacement of the timer piston caused by the impulse force is prevented, to stabilize the advance angle.
Furthermore, in the same publication, a structure is disclosed in which the timer piston is slidably inserted in a steel pipe which is internally fitted in the housing. A high pressure chamber is formed at one end of the timer piston with the low pressure chamber formed at the other end of the timer piston by closing off the two ends of the steel pipe with separate disks that are secured to the housing by bolts.
When forming the high pressure chamber with the integrated flange which, together with the steel pipe, constitutes an integrated unit, the clearance between the steel pipe and the timer piston can be maintained almost constant even if the temperature changes, by constituting the steel type and the timer piston of the same material. This is done to prevent the fuel from leaking through a clearance caused by a difference in thermal expansion that exists when different materials are used to constitute the steel pipe and the timer piston, and to greatly facilitate the seal management at the high pressure side. However, in the structure described above, in which a valve mechanism (impact valve) is provided inside the timer piston to absorb the shock wave generated in the high pressure chamber, i.e., a moving member is provided inside another moving member, there is a problem in that the movement of the timer piston is directly communicated to the valve mechanism to prevent stable movement of the valve mechanism.
In addition, in the structure described above, in which the high pressure chamber and the passage through which the fuel is supplied to the high pressure chamber are made to communicate with each other via the valve mechanism provided at the timer piston, the individual passages must be designed and machined by anticipating the range of movement of the timer piston in order to assure such a communicating state regardless of where the timer piston is positioned. This tends to make the design and machining more difficult and is likely to complicate the structure itself. Furthermore, if the valve mechanism is to be provided inside the timer piston by machining the timer piston, full consideration must be given to ensure that the timer piston does not become deformed and, at the same time, careful design is required to ensure that damage to the valve mechanism does not directly result in damage to the timer piston.
Thus, when the various factors of the structure described above are taken into consideration as a whole, the structure cannot be considered practical from a design standpoint in terms of productivity, durability, reliability and the like.
In addition, in a timer device in which the steel pipe and the flange are constituted separately (a timer device in which a high pressure chamber is formed by providing a separate disk at an opening end of the steel pipe), the seal member may become exposed to a high pressure fuel depending upon the seal structure at the high pressure chamber side, thereby risking having the seal member directly subject to the effect of the shock wave generated by the drive reactive force. In addition, if separate disks are to be secured to the housing with bolts, the mounting bolts are often provided in the vicinity of the steel pipe in order to save space and achieve miniaturization. In such a case, it is possible that the internal surface of the cylindrical hole of the housing through which the steel pipe is inserted will become distended or deformed when the bolts are tightened, which may, in turn, result in the sliding surface of the steel pipe becoming deformed and prevent the timer piston from sliding smoothly.